The present invention relates to a carrier for use in electrophotographic developers and an apparatus for producing the carrier, and more particularly to a ferrite carrier coated with a film prepared by plasma polymerization, i.e., a plasma-polymerized film.
Two-component developers comprising a toner and a carrier are used in electrophotography for developing electrostatic latent images by the cascade process, magnetic brush process or the like.
The toner contained in such a two-component developer is used for development and thereafter transferred and fixed to give copy images and is thereby consumed gradually, while the carrier is collected, recirculated and used again along with the toner.
When the carrier is repeatedly used by collection and recirculation, there arises the problem that toner particles adhere to carrier particles, impairing the characteristics of the carrier and affording copy images of lower quality.
For example, Unexamined Japanese Patent Publication No. SHO 59-53857 discloses a process for coating carrier particles with a resin such as a fluorocarbon resin to overcome the above problem.
Resin-coated carrier particles are prepared generally by blowing off carrier particles with heating in the form of a powder cloud, spraying the cloud with a coating solution of a resin in a solvent and drying the coated particles (spray-drying process), or by dipping carrier particles in a coating solution and removing the solvent by heating. These conventional processes for preparing coated carrier particles involve the problem of permitting agglomeration of carrier particles depending on the spraying condition or the amount of blow, and further the problem that the heating degrades the coated carrier substance. In fact, particles containing a low-melting point substance, such as binder-type carrier particles, can not be coated by the conventional process which involves heating.
The conventional processes have another problem in that the coated carrier particles obtained have a relatively thick coating and are uneven in the thickness of the coating. The thick coating gives rise to the problem that the carrier becomes triboelectrically charged to result in a charge buildup when repeatedly used. Furthermore, the carrier coated by the spray-drying process has the problem that some carrier particles remain locally uncoated, permitting adhesion of toner particles to the uncoated portion.
Briefly, the preparation of coated carrier involves the problem of agglomeration of carrier particles or degradation of the carrier, while the coated carrier obtained has the problem of large or uneven coating thickness or incomplete coating.